Lets talk about Raptor.

Ok, now we have some more information about this beast. The following is a summary of the discussion done in the Raptor thread in the NSF forums. All props go to the respective posters there, I am just re-posting here and collecting the info.

The engine is encased in a protective shield that wraps around the powerpack and the chamber, leaving only the nozzle (and some fuel regen plumbing) exposed. From the light texturing shown in the drawing, it seems to be an considerably thick wall of some kind of aramid composite.

Raptor Turbopump and preburners arrangement appears impressively compact. 2001 RD191 had similar dimensions, Ø1.45 nozzle (Ø240mm throat given 37:1 area ratio) with 25.8MPa chamber pressure and 89 sea level T/W, latest 2013 design update RD193 has T/W of 103. We can also see a number of integrated components, suggesting a large percentage of the engine is 3d printed. If the performance goals pan out, this is surely going to break the record for thrust to weight ratio. And it has to...this engine can make or break the ITS project (especially if you consider the payload fractions SpaceX is looking at).

The LOX pre-burner and powerpack is situated directly above the main chamber, and the pre-burner seems to be annular, wrapping around the top of the chamber. The fuel powerpack (with multi-stage pumps) and pre-burner is on the left side, and the methane is used for regenerative cooling and autogenous pressurization.

Btw, there appears to be three nozzle variants. The Vacuum engine at 200:1, the ITS Ship at 50:1 (or slightly higher which are optimized for earth landing) and the ITS Booster at 40:1 which is not optimum but necessary to fit. The released CAD pictures seem to show this and it makes sense.

All in all, this looks like an extremely compact and competent design. Looks are strictly secondary though, we will have to see if/how SpaceX will reach the design goals for this. I think that the development process is going to be long, complex and arduous before this hits all the requirements projected in the design. Happy times ahead. C:

"I think that the development process is going to be long, complex and arduous before this hits all the requirements projected in the design."

I think 3D-printing could be a game changer in terms of R&D velocity, because it allows an iterative design of unknown before granularity:

I.e. they might have intentionally created a lower pressure version that they know must be able to survive 100 bars of pressure

They now plan to iterate up the design to 300 bar pressure, by using ~15% thrust increments:

At every step they look at the data produced by the heavily instrumented engine, measure wear and tear and various other hard to simulate properties in post- hotfire inspections.

They'd be able to tell exactly at every step whether the next 15% up-rating is safe for a given component, because wear & tear, instrumentation data and computer simulations will show degradation long before a component results in actual test failure.

If a component's characteristics during a test deem it unsafe for the next 15% iteration then that component gets improved, it gets re-printed and it gets re-tested at the current pressure level. It only proceeds to the next level if all data resulting from the current pressure level deem all components safe for the pressure upgrade.

Since the dimensions don't change much the mass of a component might increase, but its placement within the engine and its interaction with other components not so much - so despite the high level of integration between components it should be relatively common that only a part of the engine has to be re-printed.

This way it would require about 8x 15% iterations to reach the full 300% pressure goal. With 1 month for each iteration and a 25% repeat rate they'd be able to finish this in 12 months.

With faster re-printing of the engine it could be even faster than that, or they could use smaller, safer increments.

BTW., I think this might also explain why there are 3 Raptor test stands:

with 3 engines being upgraded in parallel, but each upgrading a different component, testing frequency can be increased.

If they have multiple 3D printers then they can print multiple copies of the same upgraded component in parallel, without significantly increasing the length of an iteration.

If they are unsure about how to proceed with a component's upgrade they could try multiple versions in parallel

... I don't know how much (if any) of these processes they are using, but 3D printing allows very unconventional development methods!

I agree, there was video circulating a while back where one of the engineers spoke of how 3D printing has sped up testing of Merlin engine development. Instead of taking weeks or months to create or modify an existing part, all they had to do was modify a cad model, and print it out and test it. I do not have the source video. I just remember seeing it.

That's a bit of a worry with 42 engines, would it be a case of the computer going "nope" and then another attempt soon after as long as the launch window permits? I know it will have excellent engine out ability, but going through with the launch even though the only problem is that one or two didn't start seems silly.

I guess there will be a TON of static fires during the "booster testing" part of the timeline, REALLY hoping for grasshopper style testing, seems like they will have to to test the "land back on the mount" concept anyway, I don't imagine it would be a good day if the first full system test flight results in the pad being destroyed

I actually didn't understand it very well until my interest in raptor prompted me to look into it. The fact that the two preburner exhausts would still combust when they reached the main chamber because they are fuel and oxidiser rich respectively took me by surprise. I really hadn't considered that fact until it was directly pointed out to me.

They'd be able to tell exactly at every step whether the next 15% up-rating is safe for a given component, because wear & tear, instrumentation data and computer simulations will show degradation long before a component results in actual test failure.

I love the sound of a laser ignitor! Although isn't a spark actually plasma.

Odd story on static discharge via sparks. I know a company with a similar design, test, iterate ethos to SpaceX. They had a static discharge arrangement consiting of a nut running close to a plate (context being they needed to stop static discharge across a bearing surface). The nut went through three different iterations of shape before one that was durable and didn't weld it'self onto the thread. A normal nut welded it'self onto the thread. A bespoke conical nut eroded the tip in the current then stopped working (too great a distance). Finally, a domed head nut worked and lasted.

Spark ignition questions: 1) Any reasons why it would be more reliable with MethaLox over KP or H? 2) Do you think there are ways an engine-out anomaly traced back to ignition system could still allow for the same engine to be used later in flight, but before being removed and inspected? (Say on a TMI burn you have an engine fail to start, others compensate, the anomaly is traced to a failed ignition; can this engine be relit without hands-on work? Basically, I'm wondering how close to a spark plug these systems are..)

In piston engines, generally spark ignition is used with lighter weight fuels (gasoline and lighter) while compression ignition is used with heavier hydrocarbons; kerosene seems to be too heavy for use in spark-ignition engines.

in contrast, most natural gas stoves use spark ignitors and they generally work quite well even in the inherently messy environment of a cooktop.

a minute of googling found this; looks like spark ignition is typically used, but it's less time critical since combustion is continuous (rather than ignition needing to happen with precise timing as in a typical reciprocating engine) and the engines generally don't have to be started near-simultaneously (as is typical in a multi-engine rocket); you spin up the turbine with an electric motor, then inject fuel and spark it and it eventually catches to the point where you generate enough power to keep the fan turning.

Spark ignition is much easier when you have gas-phase components at the inlet. The engine probably requires a high-pressure reservoir of methane and oxygen to get started, but then it can refill those after the engine is running.

I don't think so. Even if they are planning to use high pressure gas for their thrusters, the volume is too high. The necessary pressure vessels for startup would be relatively small reservoirs meant to provide a couple seconds of fuel flow. I don't think they would put them inside the propellant tanks either, since that would require extra insulation and heating to prevent condensation.

Counterintuitively, composite pressure vessels do not benefit from a spherical design in terms of mass efficiency. Since cylindrical pressure vessels are much simpler to construct, composite pressure vessels are usually cylindrical. That means the spheres in the ITS drawings are probably not pressure vessels.